add classes for main modalities

[jjjermiah]

• starting with an ENUM to describe strings like RTSTRUCT instead of just strings
• possibly describing main types of modalities for med-imagenet

[jjjermiah]

Modality Name	Modality Tag (0008,0060)	SOP Class UID
Computed Tomography	CT	1.2.840.10008.5.1.4.1.1.2
Magnetic Resonance Imaging	MR	1.2.840.10008.5.1.4.1.1.4
Ultrasound	US	1.2.840.10008.5.1.4.1.1.6.1
Positron Emission Tomography	PT	1.2.840.10008.5.1.4.1.1.128
Digital Radiography	CR	1.2.840.10008.5.1.4.1.1.1
Mammography	MG	1.2.840.10008.5.1.4.1.1.1.2
Nuclear Medicine	NM	1.2.840.10008.5.1.4.1.1.20
Radiotherapy Structure Set	RTSTRUCT	1.2.840.10008.5.1.4.1.1.481.3
Radiotherapy Dose	RTDOSE	1.2.840.10008.5.1.4.1.1.481.2
Radiotherapy Plan	RTPLAN	1.2.840.10008.5.1.4.1.1.481.5
Secondary Capture	SC	1.2.840.10008.5.1.4.1.1.7
X-Ray Angiography	XA	1.2.840.10008.5.1.4.1.1.12.1
Mammography Tomosynthesis	MG	1.2.840.10008.5.1.4.1.1.1.3
Ophthalmic Photography	OP	1.2.840.10008.5.1.4.1.1.77.1.1
Endoscopy	ES	1.2.840.10008.5.1.4.1.1.33
X-Ray Computed Tomography	CT	1.2.840.10008.5.1.4.1.1.2.1
Enhanced MR Image	MR	1.2.840.10008.5.1.4.1.1.4.1
Enhanced US Image	US	1.2.840.10008.5.1.4.1.1.6.2
Enhanced XA Image	XA	1.2.840.10008.5.1.4.1.1.12.2
Segmentation	SEG	1.2.840.10008.5.1.4.1.1.66.4